Polycelis asiatica Selinova, 1985

Sun, Xin-Xin, Wang, Lei, Li, Ning, Sluys, Ronald, Liu, De-Zeng, Dong, Zi-Mei & Chen, Guang-Wen, 2024, Two species of Polycelis (Platyhelminthes, Tricladida, Planariidae) newly recorded for the Qinling Mountains and the Loess Plateau in China, with a comparative discussion on their karyotypes, Zootaxa 5536 (4), pp. 581-599 : 584-589

Polycelis asiatica Selinova, 1985

Material examined. ZMHNU-GYS6 , Changping village (33°43′27″N, 107°58′8″E), Fuping County, Shaanxi Province, China, alt. 1715 m above sea level (a.s.l.), 27 July 2018, coll. G-W. Chen , Z-M. Dong , P-Z. Wang and co-workers, sagittal sections on 27 slides ; ZMHNU-GYS1–4 and 11, ibid., sagittal sections on 27, 20, 26, 19 and 42 slides respectively ; ZMHNU-GYS7 and 10, ibid., transverse sections on 44 and 10 slides respectively ; ZMHNU-GYS5 and 8, ibid., horizontal sections on 22 and 26 slides respectively ; RMNH.VER.21595.1, ibid., sagittal sections on 13 slides ; RMNH.VER.21595.2, ibid., sagittal sections on 37 slides .

Habitat and reproduction. All animals were collected from a freshwater stream at Guanyin Mountain Protection Station in the central Qinling Mountains ( Figs 1 , 2A, B ) at 1715 m a.s.l. (air temperature 9 ℃, water temperature 11 ℃). Many animals were sexually mature at collection, but no cocoons were found.After 11 months under laboratory conditions, the population failed to adapt to the laboratory environment and there were no surviving individuals. Furthermore, no cocoons were laid by the sexual animals.

Karyotype. Each of the five, randomly selected specimens exhibited diploid chromosome complements. From a total of 104 metaphase plates examined, 86 chromosome complements were diploid with 2n = 2x = 40 = 30m + 8sm + 2st, in which 4 th, 7 th,12 th and 16 th chromosomes were submetacentric, 10 th chromosomes were subtelocentric and the remaining were metacentric ( Fig. 3 ). The chromosome complements of the remaining 18 plates could not be determined due to either lack of well-dispersed chromosomes or over-dispersed sets of chromosomes. Karyotype parameters, including relative length, arm ratio, and centromeric index are shown in Table 1.

Description. In living sexual specimens, the body measured 8–13 mm in length and 4–6.4 mm in width. The frontal margin of the body is slightly convex with a small protrusion in the mid-line, while it is provided with a pair of bluntly pointed auricles projecting antero-laterally ( Fig. 2C ). About 63–80 small eyes are arranged into two bands that run from the frontal margin of the body along the lateral margins to a short distance posterior to the auricles ( Fig. 2C ). Dorsal surface brown, with the ventral surface having a paler shade of brown.

The pharynx, situated in the posterior half of the body, measures about 1/4 th of the body length ( Fig. 2C ). The mouth opening is located at the posterior end of the pharyngeal pocket. Outer pharyngeal musculature is composed of a subepithelial layer of longitudinal muscles, followed by a layer of circular muscles. Inner pharyngeal musculature is composed of a thick subepithelial layer of circular muscles, followed by a thin layer of longitudinal muscles ( Fig. 4A ).

A pair of ventral ovaries are located at 1/3 – 1/5 of the distance between the brain and the root of the pharynx, occupying about 1/4 of the dorso-ventral space ( Fig. 4B ). The oviducts arise from the posterior wall of the ovaries and run ventrally in a caudal direction. At about the level of the penis, the oviducts ascend to the dorsal side of the male atrium and then unite to form a common oviduct, which opens into the dorsal portion of the small common atrium ( Figs. 4C , 5A , 6 ).

The large, sac-shaped copulatory bursa, which occupies almost the entire dorso-ventral space, is lined by stratified columnar epithelium with basal nuclei ( Figs. 4D , 5A, B, D , 6 ). The bursal canal arises from the postero-dorsal wall of the copulatory bursa, then runs in a caudal direction, curves downwards and, subsequently, communicates with the common atrium ( Figs. 5A, B , 6 ). The bursal canal is lined with cylindrical, ciliated cells with basal nuclei and is surrounded by a subepithelial layer of circular muscle, followed by a thin layer of longitudinal muscles ( Fig. 4C ). The ventral portion of the bursal canal, close to its communication with the common atrium, is surrounded by a slightly thicker coat of muscles, a feature that was well-discernible in several specimens (e.g., specimen ZMHNU-GYS6), but much less clear in others (e.g., specimen RMNH.VER.21595.2).

The ventrally located testes are well-developed, occupying about one-half of the dorso-ventral space, but sometimes also extending beyond the mid-line of the body (i.e., a little bit more to the dorsal surface) (e.g., specimen ZMHNU-GYS4) ( Fig. 4B ). There are about 40–50 testes in sexually mature individuals that extend from the level of the ovaries to the root of the pharynx. Mature spermatozoa are present in the testicular follicles and the vasa deferentia ( Fig. 5C ). In all specimens examined, the two vasa deferentia enlarge to form spermiducal vesicles on either side of the posterior part of the pharyngeal pocket and decrease in diameter before bending sharply towards the dorsal body surface. Hereafter, the sperm ducts penetrate the dorso-lateral wall of the penis bulb, then exhibit a knee-shaped bend towards the ventral surface and, subsequently, open separately and symmetrically into the anterodorsal portion of the large seminal vesicle ( Figs. 5B–D , 6 ). The sperm ducts are lined with nucleated cells and are surrounded by a layer of circular muscles.

The penis bulb is composed of intermingled muscle fibres; this coat of musculature extends onto the basal part of the penis papilla, almost up to the dorsal wall of the seminal vesicle ( Figs. 5A–D , 6 ). The seminal vesicle is lined by a thick, nucleated epithelium. The dorsal wall of the seminal vesicle is lined with a rather thick epithelium, as compared with that of the ventral wall; the dorsal wall of the seminal vesicle forms a number of distinct, or even large, glandularized folds; ( Figs. 5A–F , 6 ).

The more or less bulbous penis papilla, which protrudes from the dorsal wall of the male atrium, is asymmetrical, in that the large seminal vesicle is ventrally displaced and almost touches the wall of the penis papilla, thus leaving only an extremely thin ventral lip ( Figs. 5A–D , 6 ). The seminal vesicle communicates with a narrow and short ejaculatory duct, which traverses the slender and flexible tip of the penis papilla, opening at the very end of the latter ( Figs. 5C, F , 6 ). The common atrium is small and communicates with the male atrium via a rather narrow and short channel. The gonoduct receives the openings of abundant cement glands ( Fig. 6 ).

Discussion. The anatomy of the copulatory apparatus of our specimens from the Qinling Mountains closely resembles that described for Polycelis asiatica from Western Pamir by Selinova (1985). Particularly striking similarities concern (1) the large, ventrally displaced seminal vesicle of which the dorsal wall is provided with glandularized folds, (2) the strong, intermingled coat of muscle on the penis bulb that extends to the seminal vesicle; (3) the thin and flexible tip of the penis papilla; and (4) vasa deferentia that exhibit an intrabulbar knee-shaped bend towards the ventral surface before opening separately and symmetrically into a large seminal vesicle. These similarities concern detailed structural features that constitute key characters in the identification and recognition of the species. As such, these characters carry greater taxonomic weight than features known to be more variable within species. Furthermore, in the weighting and evaluation of taxonomic characters, it should be considered that, thus far, Selinova’s (1985) publication was the only primary study dealing with the species P. asiatica . Given all of this, it is not surprising that there are also some differences between Selinova’s (1985) account of the species and the description of our material.

For example, in Selinova’s (1985) specimens the dorsal surface was black, with the ventral surface being light grey. In contrast, the Chinese specimens exhibited a brown dorsum and a light brown ventral surface. However, it is well-known that in species of Polycelis colouration can be rather variable. This is exemplified by P. coronata , in which the colour of the dorsal side ranges from light brown to black ( Kenk 1973).

Then, Selinova (1985) mentioned that the largest individuals from Tajikistan had 76–78 eyes on either side of the head, while smaller animals showed 46 eyes and immature specimens 19–22 eyes on either side of the body. In our specimens, eye number ranged between 63–80. However, eye number in Polycelis is known to vary and generally increases during the growth of an animal ( Aikawa & Shimozawa 1991), as is evident also from Selinova’s (1985) data and our specimens.

In the specimens from Tajikistan, the testes were reported as completely occupying the dorso-ventral space of the body, whereas in our animals from China, the ventrally located follicles occupy about one-half of the dorso-ventral space, but sometimes extend also beyond the mid-line of the body.

In the specimens from Tajikistan, the proximal section of the bursal canal, closest to the gonopore, would receive the cyanophilic secretion of glands. We have not observed similar glands in our specimens from China. However, this difference may simply be due to the different staining techniques applied, as Selinova (1985) applied Heidenhain’s Azan stain, while we used Hematoxylin-Eosin. The former stain provides much more optimal differentiation of the various tissues, in contrast to Hematoxylin-Eosin, which notably restricts differentiation of secretory elements.

One feature mentioned by Selinova (1985) for the specimens from Tajikistan deserves somewhat more detailed consideration and evaluation, viz., the presence of a kind of muscular sphincter on the proximal section of the bursal canal, near its junction with the common atrium. From Selinova’s account, one can only conclude that this presumed sphincter is only minimally developed. In the precise reconstruction of figure 2b, it is hardly different from the other musculature surrounding that portion of the ejaculatory duct. That is likely why the author indicated it more prominently in the much more schematic figure 2a, although, again, it does not look like a strongly developed sphincter. In contrast, Dyganova & Porfirjeva (1990, fig. 44) depicted a strong, bulbous sphincter, resembling that of Polycelis felina . However, as these authors did not examine any new material but solely relied on Selinova’s (1985) account, their interpretation of the sphincter must be rejected. As described above, in some of our specimens from China the musculature on the most proximal part of the bursal canal, near its communication with the common atrium, was only slightly thicker than on the rest of the canal, whereas in other specimens this was much less clearly developed. We interpret this situation as corresponding with Selinova’s (1985) original account.

As Selinova (1985) did not report on the chromosomes, the present study is the first account on the chromosome portrait of P. asiatica . Its diploid complement with 30 metacentric chromosomes, 8 submetacentric chromosomes, and 2 subtelocentric chromosomes, represents the first species of Polycelis for which subtelocentric chromosomes have been described.

Presently known localities for P. asiatica are rather far apart as the type specimens came from the Khabu-Rabot Pass , 270 km from Khorog , Pamir Mountains, Tajikistan (at an altitude of 2500 m a.s.l.), while our specimens were found in the Qinling Mountains, China (alt. 1715 m a.s.l.) .

The features of P. asiatica invite comparison with three other species, viz., P. pamirensis de Beauchamp, 1961 , P. pathan de Beauchamp, 1959 , and P. xigazensis Liu, 1993 . The last-mentioned species shares with P. asiatica the following features: vasa deferentia showing an intrabulbar knee-shaped bend towards the ventral body surface, thereafter opening into the dorsal section of the seminal vesicle; the latter is ventrally displaced, thus making up a rather narrow ventral lip on the penis papilla; dorsal wall of the seminal vesicle lined with a relatively thick epithelium, as compared with the epithelium of the ventral wall ( Liu 1993, p. 179, fig. 4); intermingled musculature of the penis bulb extending up to the seminal vesicle. In fact, a good case could be made for considering P. xigazensis a junior synonym of P. asiatica .

Although for P. xigazensis a flexible tip of the penis papilla was not specifically described in these terms, the description mentions that the dorsal lip of the papilla may be ventrally curved or be extended (see also Liu 1994, Pl. I, fig. 6). Thus, it would be interesting to check this on the type material. It is noteworthy that the seminal vesicle of P. xigazensis may be much larger than the reconstruction drawing suggests ( Liu 1994, fig. 4) and may occupy most of the penis papilla (cf. Liu 1994, Pl. I, fig. 5). The lining of the dorsal wall of the seminal vesicle consists of columnar, glandular epithelium ( Liu 1993); whether this is accompanied by distinct folds remains to be checked on the type material.

A flexible tip of the penis papilla is present in P. pathan and P. pamirensis , both from Afghanistan ( de Beauchamp 1959, 1961; Ichikawa & Kawakatsu 1966), but both species are clearly different from P. asiatica . Although P. pamirensis also has a highly muscularized penis bulb, the seminal vesicle is not ventrally displaced. The vasa deferentia do not display the knee-shaped bend, as in P. asiatica , but enter the antero-lateral portion of the vesicle ( de Beauchamp 1961, Ichikawa & Kawakatsu 1966). In P. pathan , the entire penis papilla is a rather thin, elongated and flexible structure, while a very large and elongated intrabulbar seminal vesicle lies fully recessed in the parenchyma. The dorsal portion of the vesicle is covered by a very thick coat of intermingled bulbar muscle fibres, whereas there is hardly any musculature on its ventral side ( de Beauchamp 1959).

Figures

FIGURE 1. Location of collection sites of Polycelis in Shaanxi Province, China.

FIGURE 2. Habitat and external appearance of Polycelis asiatica. A & B sampling site and habitat. C sexually mature, live individual. D Head with eyes, Scale bar: 500 μm.

FIGURE 3. Polycelis asiatica. A metaphase plate of diploid complement. B karyogram of diploid complement. C idiogram. Scale bar: 5 μm.

FIGURE 4. Polycelis asiatica. Photomicrographsof sagittal sections.A RMNH.VER.21595.2, showing pharyngealmusculature. B sagittal section of ZMHNU-GYS1, showing ovary and testes. C ZMHNU-GYS6, showing common oviduct and longitudinal muscles and circular muscles around bursal canal. D ZMHNU-GYS5, showing large copulatory bursa. Scale bars: 100 μm.

FIGURE 5. Polycelis asiatica. Photomicrographs. A sagittal section of RMNH.VER.21595.2, showing penis papilla, copulatory bursa, bursal canal, ejaculatory duct and seminal vesicle. B sagittal section of ZMHNU-GYS6, showing copulatory bursa, vas deferens, ejaculatory duct and seminal vesicle. C sagittal section of ZMHNU-GYS6, showing spermiducal vesicle, ejaculatory duct and seminal vesicle. D sagittal section of ZMHNU-GYS4, showing vas deferens, gonopore, ejaculatory duct and seminal vesicle. E transverse section of ZMHNU-GYS7, showing seminal vesicle, bursal canal and common oviduct. F horizontal section of ZMHNU-GYS8, showing ejaculatory duct and seminal vesicle. Scale bars: 100 μm.

FIGURE 6. Polycelis asiatica. Sagittal reconstruction of the copulatory apparatus of ZMHNU-GYS6. Scale bar: 100 μm.